In the production of a seamless tube on a mandrel mill, first, a heated solid billet is pierced by a piercer to make a hollow shell. Subsequently, after insertion of a mandrel bar into this hollow shell, the shell is usually subjected to elongation-rolling on a mandrel mill consisting of five to eight roll stands. As a result of this elongation rolling, the shell wall thickness is adjusted to a prescribed wall thickness and the shell is subjected to circumference working where the outside diameter thereof is decreased to cause the circumference length thereof to be reduced. After the completion of elongation rolling, the mandrel bar is withdrawn/stripped. Thereafter, the elongation-rolled shell is subjected to rolling by means of a stretch reducer to have a prescribed outside diameter, yielding a seamless tube as a product.
In the past, there has been frequently used a 2-roll mandrel mill which has a plurality of roll stands, the roll stand each having a pair of opposite grooved rolls as a roll pass. In this 2-roll mandrel mill, the paired grooved rolls in the adjacent roll stand are shifted so as to be oriented by 90-degree with respect to drafting/reduction-rolling direction, relative to that of a reference roll stand.
Furthermore, in some mandrel mills, used is a 3-roll mandrel mill in which three grooved rolls are arranged as a roll pass in each roll stand, with the orientation of reduction-rolling direction being 120°, or a 4-roll mandrel mill in which four grooved rolls are arranged in each roll stand, with 90° orientation as the reduction-rolling direction.
In the production of seamless tubes by this mandrel mill, when the rolling conditions on the mandrel mill are not appropriate, fin flaws attributable to overfill and through-wall defects attributable to underfill may occur. Various proposals have hitherto been made in order to prevent these fin flaws and through-wall defects.
For example, Patent Literature 1 proposes a mandrel mill which increases the elongation ratio of tube (in other words, increases the manufacturing efficiency of seamless tubes) and can prevent the occurrence of through-wall defects attributable to underfill and a method of producing seamless tubes using this mandrel mill According to this proposed method, it is claimed that by using a 2-roll mandrel mill in which the roll diameter ratio (groove-bottom roll diameter divided by section diameter of elongation-rolled shell) of the first roll stand and the second roll stand is set at not less than 4.6 and a 3-roll mandrel mill in which the roll diameter ratio is set at not less than 2.8, it is possible to increase the manufacturing efficiency of seamless tubes without causing rolling defective.
Patent Literature 2 proposes a method of rolling seamless tubes which can prevent the occurrence of through-wall defects that pose a problem in the rolling of thin-walled tubes on a mandrel mill. According to this method, it is claimed that by setting the ratio of DF:roll diameter DF of groove bottom of rolling mill roll to RI:radius of curvature of groove bottom of rolling mill roll in at least one stand (preferably #2 stand, more preferably all stands) (hereinafter also referred to merely as the “ratio of RI/DF”) at not less than 0.275, it is possible to produce thin-walled tubes by preventing the occurrence of through-wall defects.
Patent Literature 3 proposes a mandrel mill rolling method which can effectively prevent both overfill and underfill. This is a method which specifies the outside diameter of the shell and the ratio of groove caliber circumference length to finishing circumference length in the first stand and second stand in prescribed ranges for shells made of various steel grades such as ordinary carbon steel and alloy steel and having various wall thicknesses. According to this method, it is claimed that even by using the combination of rolling rolls made of solely one kind of groove caliber, it is possible to effectively prevent the occurrence of fin flaws attributable to overfill and hard-stripping (mandrel withdrawing) attributable to underfill as well as the occurrence of flaws attributable to hard-stripping.
In the methods of producing seamless tubes described in Patent Literatures 1 to 3, the occurrence of flaws and defects attributable to underfill and overfill is prevented by adjusting the roll diameter ratio, the ratio of RI: radius of curvature of relevant roll groove caliber to DF: groove-bottom roll diameter (i.e., RI/DF) or the ratio of circumference length of roll groove caliber to finishing circumference length.
However, because in multi-roll mandrel mills (3-roll or 4-roll mandrel mills) the profile of roll pass is subjected to geometric restrictions, the prevention of underfill and overfill with the above-described adjustment of the roll diameter ratio, RI/DF or the ratio of circumference length of groove caliber to finishing circumference length may be limited.
In addition, in designing the roll diameter ratio, it is necessary to control the roll diameter ratio in a given range in order to prevent the occurrence of flaws and defects attributable to underfill and overfill. However, when an appropriate roll diameter ratio is to be applied for controlling, this may be restricted by the structure of a roll housing.
FIGS. 1A and 1B are diagrams to explain an example of the case where designing the roll diameter ratio in a 3-roll mandrel mill is restricted. FIG. 1A schematically shows the arrangement of grooved rolls in an ordinary roll stand, and FIG. 1B schematically shows the side surface of a roll chock portion.
As shown in FIG. 1A, each roll stand is provided with three grooved rolls to constitute a roll pass 6, the roll each comprising a grooved roll body 1, a roll shaft 2 and a roll chock portion 3 that are integrated. The orientation of elongation rolling direction of each of the grooved roll bodies 1 is 120° apart. As shown in FIG. 1B, the roll chock portion 3 is composed of a bearing 4 and a bearing box 5.
In the roll stands of the 3-roll mandrel mill illustrated in FIG. 1, in particular, in the case where the elongation rolling of small-diameter tubes is performed, if the grooved roll diameter (the diameter of the grooved roll body) is designed to be small, an ordinary design of a roll housing encounters the interference of bearing boxes of the roll chock portions 3 with each other. For this reason, it is impossible to dispose the roll bodies 1 in a closer relation to each other, resulting in the occurrence of fin flaws.
In a multi-roll mandrel mill, the more the number of rolls, the more fin flaws are liable to occur. It seems that this is because the more the number of rolls, the narrower the high surface pressure region in the outer surface of the shell during rolling becomes in a width-wise (circumferential) direction, compared to that in the rolling direction, with the result that the metal flow in the rolling direction is constrained to facilitate the flow to occur in a width-wise (circumferential) direction.
In the case where a small-diameter tube is made on a multi-roll mandrel mill having a number of rolls, the fin flaw due to overfill becomes large. In particular, when the same reduction-rolling amount/draft as the case of a large-diameter tube is adopted, the reduction rate of outside diameter increases, facilitating the occurrence of fin flaws.
In a multi-roll mandrel mill which has 3 rolls, a gap adjustment between the arranged three grooved rolls is limited due to the interference of the roll chock portions. For this reason, the challenge is to control the roll diameter ratio in an appropriate range by eliminating the mutual interference between the roll chock portions so as to enable the underfill and overfill to be efficiently prevented.
In general, a multi-roll mandrel mill is effective in preventing defectives as through-wall flaws in tube, and in reducing wall thickness eccentricity, and the like. This effect is especially remarkable in thin-wall alloy steel tubes, and can increase productivity because a high elongation ratio can be ensured. For this reason, while a 2-roll mandrel mill is frequently used, a multi-roll mandrel mill which has more than 2 rolls, such as 3 or 4 rolls, is also used in some cases.
However, a multi-roll mandrel mill has the problem that designing the roll diameter ratio required for preventing underfill and overfill is limited. For this reason, the advantage of a multi-roll mandrel mill is not necessarily sufficiently exploited.